Baby Star Caught in the Act of Growing

Illustration of a young protostellar system with matter from the surrounding cloud streaming into the disk. The zoom shows the rotating disk surrounding the young protostars and the accretion of matter from the surrounding cloud.
(Bill Saxton/NRAO)

Images of the protostar taken with the Spitzer Space Telescope (top) and the Gemini North telescope (bottom). The contours outline the millimeter-wave emission as observed by the Submillimeter Array (SMA). The top image shows reflected light as
(John Tobin)

Beneath a dusty disk of creation, a baby star's mass has been measured for the first time.

The star, called L1527 IRS, is only one-fifth the mass of the sun, and is expected to keep growing as the swirling disk of matter surrounding it falls into its surface.

Astronomers estimated the star formed around the same time that Neanderthals evolved on Earth: just 300,000 years ago.

In fact, if L1527 had grown in mass just a bit more quickly in its earlier years, it could be as young as 150,000 years old. Either way, the star is among the youngest discovered in the universe, said lead researcher John Tobin.

"There's five times more material surrounding it that could be incorporated in [the star]," said Tobin, a Hubble fellow at the National Radio Astronomy Observatory in Virginia. "There is still a lot of room to grow, so to say."

Over millions of years, material falls on the protostar and releases quite a bit of energy. In L1527, 90 percent of its energy comes from material landing on the surface of the protostar. The remaining 10 percent comes from the star itself.

"There is a rotationally supported disk around this protostar," said Tobin, adding it's a "key element" in building planets. "It lets the material hang out long enough for the planet formation process."

But it's far too early in L1527's evolution to talk about protoplanets, he added. In the scale of stellar evolution, the star is at Stage 0. By comparison, Earth's sun is a middle-aged matron, at 4.6 billion years of age.

Mass measurements

Astronomers measured L1527's mass through simple Newtonian physics: They determined the mass by calculating the speed of the matter swirling around the protostar at a given distance.

Tobin, who has been watching this star for several years, said L1527 could serve as a useful goalpost in humans' understanding of stellar evolution.

It appears that 90 percent of young stars that are less than 1 million years old have disks of matter surrounding them. Save for the few in multiple-star systems who see their disks blown away, it appears disk formation is a universal process in a young star's life.

"We want to get a more detailed view of the structure of the rotating disk," he said, adding, "We're also trying to look at more young protostars to find more disks like this. You can get a big picture view of everything that’s going on."

The research will be published in the Dec. 6 issue of the journal Nature and includes collaborations with astronomers in the United States, Germany and Mexico.